1. Field of the Invention
The present invention relates to a semiconductor device and to a method of fabricating a semiconductor device, and more particularly it relates to a semiconductor device and method of fabricating same which eliminates over-etching and achieves stable transistor characteristics.
2. Background of the Invention
In an interconnect technology making use of self-alignment process to form contacts in a semiconductor device, and particularly a method of forming a self-aligning contact using an etching stopper made of a silicon nitride film was accompanied by the following problems in the past.
FIG. 8 shows cross-section views of process diagrams illustrating the application of a general self-aligning contact process to a memory device.
As shown in FIG. 8(a), a gate electrode 403 and an insulation film 404 made of a silicon nitride film are formed on a silicon substrate 401 and an element separation region 402, and then a silicon nitride film 405 serving as an etching stopper is deposited so as to cover the gate electrode and the silicon substrate. Then after forming a BPSG film 406 as an interlayer insulation film, self-aligning contact etching is done to form a contact plug 410. Then, after forming an interlayer insulation film 411 using a CVD process, a contact pattern is formed on photoresist 412, this being used as a mask to etch the BPSG film 406.
After the above, as shown in FIG. 8(b), the silicon nitride film 405 is etched. When doing this, if the usual nitride film etching is done with a fluorocarbon gas such as CF.sub.4, because it is not possible to achieve a selection ratio with respect to the silicon substrate, the silicon substrate 401 is simultaneously etched.
As shown in FIG. 9, which is a simplified drawing of a transistor structure, each transistor on a p-type silicon substrate 414, for example, is separated by a field oxide film 415 and, the transistor is formed by a gate electrode 416, an n-type diffusion layer 418 that will serve as a source and drain, and a contact electrode 417, when the n-type diffusion layer made of silicon is over-etched, there is current leakage in the direction A indicated in FIG. 9, the result being that it is not possible to achieve stable transistor characteristics.
While the above applies to the case in which an insulation film 404 over the gate electrode is a silicon nitride film, even in the case in which this insulation film 404 over the gate is a silicon oxide film, for the same reason as described above, the field oxide film 415 is etched, in which case there is current leakage in the direction B or the direction C indicated in FIG. 9, the result being that it is not possible to achieve stable transistor characteristics.
In the Japanese Unexamined Patent Publication (KOKAI) No. H9-205185, there is a method for removing the silicon nitride film from over the gate beforehand. This method, however, requires an extra lithography process, thereby causing the problem of an increased number of process steps.
Accordingly, in view of the above-noted drawbacks in the prior art, it is an object of the present invention to provide a novel method for fabricating a semiconductor device, whereby over-etching is prevented without the need to increase the number of lithography steps, and which achieves stable transistor characteristics.